It is known that classic antihistamines, such as mepyramine, are capable of antagonizing some effects of histamine mediated by H.sub.1 -receptors. However, these compounds have no effect on gastric acid secretion which is instead affected by other antihistaminic agents defined by Black et al. (Nature 236, 385, 1972) as histamine H.sub.2 -receptor antagonists. This has indicated that another kind of receptors (H.sub.2) already described by Ash and Schild (Brit. J. Pharmacol. Chem. Ther. 1966, 27, 427-39), is involved in the gastric secretory response which is not blocked by the conventional antihistamines of the H.sub.1 -type.
Examples of H.sub.2 -receptor antagonists capable of antagonizing gastric acid secretion include burimamide, metiamide, and cimetidine. More recently, new H.sub.2 -antagonists, such as rantidine (Bradshaw et al., Brit. J. Pharmacol. 66, 464P, 1979), tiotidine (P. O. Jellin, Life Sci. 25, 2001, 1979) and BL 6341 (Cavanagh et al., Fed. Proc., 40, 2652, 1981) have been discovered. These compounds are effective H.sub.2 -blockers capable of antagonizing gastric acid secretion to a greater extent than cimetidine and congeners. In copending U.S. application Ser. No. 465,572, filed Feb. 10, 1983, now U.S. Pat. No. 4,548,944, and U.S. Pat. No. 4,386,099 we have described new classes of histamine H.sub.2 -antagonists, namely imidazolyl-phenylamidines and guanidino-heterocyclylphenylamidines, which are potent H.sub.2 -blockers and active antagonists of gastric acid secretion. These compounds do not resemble the so far known H.sub.2 -antagonists, such as cimetidine, ranitidine, etc., and are characterized by a phenylformamidine grouping bearing variously substituted imidazolyl- and guanidino-heterocyclyl rings.